Imaging systems are generally of two types, using reflective and refractive or catadioptric optics. Reflective imaging systems have no chromatic aberration, and their operating spectral bandwidth is limited by the reflectance of the surfaces of said optics. These systems, however, are not appropriate for all imaging tasks. Refractive or catadioptric systems, on the other hand, are limited by the spectral characteristics of optical materials used. This includes their transparency range, and also the variation in their dispersion properties, which affects chromatic aberration correction.
The problem of chromatic aberration is well known in the field of optical imaging. Any refractive element placed in the path of a polychromatic light, gives rise to chromatic aberrations as a result of the wavelength dependence of the dispersive properties of the material making up the refractive element. For example, calcium fluoride is transparent from the ultraviolet to the thermal infrared. However, in the ultraviolet it is a relatively low-dispersion material, in the visible range it is a very low-dispersion material, and in the 3-5 μm range it is a very high-dispersion material. Zinc selenide is transparent from the mid-visible range to the thermal infrared. In the long-wave visible range it is a very high-dispersion material, in the 3-5 μm range it is a low-dispersion material, and in the 8-12 μm range it is again a high-dispersion material. The design of a refractive broadband spectral system is thus limited by the availability of optical material combinations which correct chromatic aberrations over the spectral range.
Chromatic aberration is typically caused by an imaging lens not focusing different wavelengths of light onto the exact same focal plane (the focal length for different wavelengths is different) and/or by the lens magnifying different wavelengths differently. These types of chromatic aberration are known as “Longitudinal Chromatic Aberration” and “Lateral Chromatic Aberration” respectively and can occur concurrently. The amount of chromatic aberration depends on the dispersion of the glass. Thus for example a refractive lens will focus light of different wavelengths at different focal planes. In the case of a convex convergent lens, the shorter the wavelength of the light the closer its focal plane is to the lens. Chromatic aberration is visible as color fringing around contrasty edges and occurs more frequently around the edges of the image frame in wide angle shots.
FIG. 1A illustrates the chromatic aberration effects upon a polychromatic light flux 100 caused by a simple lens 110. The passage of three light components 101, 102 and 103 of three different wavelengths, respectively, is shown. Short wavelength blue light component 101 is dispersed most strongly by lens 110 and has a shorter focal length than green light component 102 which is focused upon a detector 150. Longer wavelength red light component 103 has a longer focal length. Thus, an image formed upon detector 150 placed in the focal plane of the green light will produce an image with chromatic fringing.
Various methods are known for correcting this effect. The most common method used is to employ an achromatic lens. An achromatic lens is a couplet of two lenses made from different materials such as crown glass and flint glass which have different dispersive properties. It is possible to combine a converging lens of one material with a diverging lens of a second material such that any two wavelengths are brought to focus upon the same focal plane.
FIG. 1B shows the operational principles of an achromatic lens 120 which is configured as a couplet of two lens elements 120A and 120B made from different materials such as crown glass and flint glass. Here the correction has lengthened the focal length for a blue light component 101 and shortened the focal length for a red light component 103 so that they share the same focal length. A green light component 102 is focused in front of a detector 150, resulting in reduced but not eliminated chromatic fringing.
Over the visible light range, the best correction is considered to be obtained where the condition, V1f1+V2f2=0, is satisfied, where V1 and V2 are the Abbe numbers of the first and second lens respectively, and f1 and f2 are the focal lengths of light at wavelength 589.2 nm for the first and second lenses. This condition ensures that blue light of wavelength 486.1 nm and red light of wavelength 656.3 nm will share the same focal length. Other wavelengths of light will have similar but not identical focal lengths thus reducing the effect of chromatic aberration but not removing it all together.
Chromatic correction can be further improved by the use of an agent such as fluorspar which can be introduced forming aprochromatic lens triplets. Such combinations can be adapted such that three or four separate wavelengths can be brought into focus at the same focal lengths.
FIG. 1C illustrates the operation principles of an apochromatic lens 130 to correct for all three wavelengths of light, such that the paths of blue 101, green 102, and red 103, light components are coincident. It should, however, be noted that intermediate wavelengths (not shown here) will not focus upon exactly the same focal plane, thus the image will still display a certain degree of fringing.
One further method of chromatic correction is the use of holographic or diffraction lenses. Diffraction lenses typically deflect longer wavelengths further than shorter wavelengths of light, which is the opposite effect to that of refractive lenses. Thus, by etching a converging diffraction lens directly onto the surface of a converging refractive lens, it is possible to compensate for chromatic aberrations in a fashion similar to that described above.
The methods described above all correct for chromatic aberration over relatively narrow bandwidth, that is to say the range of wavelengths for which chromatic aberrations are corrected is limited. Current broadband imaging systems often employ reflective elements, which are intrinsically achromatic. These systems however are not appropriate when a large relative aperture or field of view is required.